1. Field of the Invention
The present invention relates to nonazide gas generating compositions that rapidly generate gases upon combustion for use in inflating occupant safety restraints in motor vehicles. Specifically, the invention relates to thermally stable nonazide gas generants capable of self-sustained burning at ambient pressures and temperatures, and exhibiting a relatively high gas volume to solid particulate ratio upon combustion at acceptable flame temperatures, as well as exhibiting a tailorable burning rate and a higher melting point than prior art formulations.
2. Background Art
Nonazide gas generant compositions have been used in recent years to replace azide based gas generant compositions. There are a number of advantages of nonazide gas generant compositions over azide gas generants, which are well documented in the patent literature, for example, U.S. Pat. Nos. 4,909,549; 4,948,439; 5,197,758; 5,531,941; 5,545,272; 5,756,929, and WO 98/04507, the content of which are incorporated by reference. Nonazide gas generant compositions are advantageous for providing a relatively nontoxic gas which is rapidly generated upon combustion. One of the disadvantages of nonazide gas generant compositions is the amount of solid combustion products, as well as the physical characteristics of the solid combustion products, formed during combustion.
As well as fuel constituents, gas generant compositions may contain other ingredients such as oxidizers, to provide the required oxygen for rapid combustion and to reduce the quantity of toxic gases generated, and catalysts to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. Therefore, gas generant compositions may also contain slag-forming constituents to cause the solid liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives such as burning rate enhancers, ballistic modifiers and ignition aids may also be used to control the ignitability and combustion properties of the gas generant composition.
For the oxidizer constituent, the use of phase stabilized ammonium nitrate (PSAN) is desirable because it results in formation of substantially all gaseous reaction products, discounting, of course, the minimal solids resulting from the use of the phase stabilizer.
The majority of gas generant compositions comprised of ammonium nitrate, however, have burn rates less than desirable for use in inflators for airbags. To be useful for passenger restraint inflator applications, gas generant compositions generally require a burn rate of at least 0.40 inch/second (ips) at 1,000 pound per square inch pressure (psi). Gas generant compositions with burn rates of less than 0.40 ips at 1,000 psi do not ignite reliably and often result in "no-fires" when tested at -40.degree. F. in the inflator.
However, in addition to producing abundant gases and minimal solids, gas generants for automotive applications should be thermally stable when aged for 400 hours or more at 107.degree. C. The compositions must also retain structural integrity when cycled between -40.degree. C. and 107.degree. C. The melting point is also important because an increased melting point will give a particular gas generant an increased margin of safety. A low melting point composition has an inherently decreased safety factor. Accordingly, many nonazide propellants based on ammonium nitrate cannot meet requirements for automotive applications.
U.S. Pat. No. 5,545,272 to Poole discloses the use of gas generant compositions consisting of nitroguanidine (NQ) at a weight percent of 35%-55%, and phase stabilized ammonium nitrate (PSAN) at a weight percent of 45%-65%. NQ is a generally desirable fuel because it generates abundant non-toxic gases when formulated with the PSAN to provide the proper oxygen to fuel balance. Poole notes, however, that the use of PSAN or pure ammonium nitrate (AN) is a problem since many gas generant compositions containing this oxidizer have unacceptably low melting points and are thermally unstable. There is also no mention of high bulk density nitroguanidine (HBNQ). Although Poole combines NQ and PSAN in the percentages given to provide allegedly thermally stable gas generant compositions, Poole reports burn rates of only 0.32-0.34 ips at 1,000 psi. Burn rates below 0.40 ips at 1,000 psi are generally not as desirable for use within an inflator due to the rapid reaction times required for properly inflating an airbag. In addition, there is no inference of the capabilities for self-sustained combustion at ambient pressure and temperature for these formulations.
U.S. Pat. No. 5,531,941 to Poole teaches the use of PSAN, and two or more nonazide fuels provided in specified groups. In view of the recognition by Poole of the above noted low burn rates, Poole combines PSAN with a fuel component containing a majority of triaminoguanidine nitrate (TAGN), and, if desired, one or more additional fuels. The addition of TAGN increases the burn rate of ammonium nitrate mixtures. TAGN, however, is a sensitive explosive that poses safety concerns in processing and handling, and is classified as "Forbidden" by the Department of Transportation, which complicates raw material requirements.
The gas generant compositions described in Poole et al., U.S. Pat. Nos. 4,909,549 and 4,948,439, use tetrazole or triazole compounds in combination with metal oxides and oxidizer compounds (alkali metal, alkaline earth metal, and pure ammonium nitrates or perchlorates) resulting in a relatively unstable generant that decomposes at low temperatures. Both patents teach the use of BKNO.sub.3 as an ignition aid.
Lund et al., U.S. Pat. No. 5,197,758, describes gas generating compositions comprising a nonazide fuel that is a transition metal complex of an aminoazole, and, in particular, copper and zinc complexes of 5-aminotetrazole and 3-amino-1,2,4-triazole that are useful for inflating air bags in automotive restraint system, but generate excess solids.
U.S. Pat. No. 5,756,929 to Lundstrom et al. relates to nonazide gas generating compositions that contain fuels selected from guanidine, azole, and other high nitrogen aliphatic, aromatic, and/or heterocyclic compounds. There is no mention of specially processed high bulk density nitroguanidine (HBNQ). Other materials may also be added to the compositions for processing, such as ignition aids, ballistic enhancers, particulate reducers and scavengers. However, the use of ammonium nitrate is not specifically described in the Lundstrom et al. patent.
In addition, Khandhadia et al., WO 98/04507 describes nonazide gas generant compositions incorporating a combination of NQ, one or more nonazide high-nitrogen fuels, and PSAN or similar nonmetallic oxidizer. Again, there is no mention of the use of specially processed high bulk density nitroguanidine (HBNQ). The gas generant compositions are disclosed to result in a good yield of gaseous production per mass unit of gas generant upon combustion and a reduced yield of solid combustion products, with acceptable burn rates, thermal stability, and ballistic properties. However, these compositions do not exhibit self-sustained combustion at ambient pressure and temperature.
Based on the above, the need remains for a nonazide gas generant composition that can be used in inflation devices and that is capable of self-sustained burning at ambient pressures and temperatures, while exhibiting a relatively high gas volume to solid particulate ratio upon combustion at acceptable flame temperatures, as well as exhibiting a tailorable burning rate and a higher melting point than prior art formulations.